The use of flow sensors for measuring the velocity of fluid flow in closed conduits is well known. Typical flow sensors include U.S. Pat. No. 4,404,860 issued to Wood et al., 4,936,151 issued to Tokio and 4,656,873 issued to Stewart. These sensors utilize various methods for converting various phenomena associated with the dynamics of fluid flow into conveniently measured analogs thereof. These analogs, by calibration, can be used to measure the rate of the fluid flow velocity. One major method of measuring fluid velocity is to determine the rotational velocity of rotary devices driven by the fluid, utilizing electro mechanical means to determine that rotational velocity, as an analog of the average velocity of the fluid. Conventional flow sensors of this type used in determining flow rate are characterized by a turndown ratio. This ratio is defined as a measure of the dynamic range of response of a given sensor over which the accuracy of the sensor output, referred to as the average actual flow rate in the pipe, is within a specified limits. Typical flow sensors of the impeller type have turndown ratios of 10:1 to 40:1. For example, if a flow sensor is able to measure flow velocity ranging from 1 ft./sec. to 25 ft./sec, then its turndown ratio is 25:1. The flow rate of the fluid is linearly proportional to the rate of rotation of the above mentioned electro mechanical means such as an impeller within this range.
Having a low turndown ratio does not provide the necessary versatility to a flow sensor. A low ratio results from the inability of a flow sensor to measure flow rate that is substantially higher or substantially lower than the normal or average flow rate. A flow sensor with a low turndown ratio would appear to measure these flow rates but the accuracy of these readings would be suspect.
Existing impeller type flow sensors typically contain impellers having a broad range of diameters. Large diameter impellers, as a result of their size, cannot be used for measuring flow rate in pipes with small diameters and typically have high rotatory inertia values. The high values increase the response time of these impellers to changes in flow. However, they offer convenience in transduction methods and, frequently, savings in the costs of producing both the impeller and its associated transduction system. A small diameter impeller, on the other hand, generally can be used in smaller pipe sizes, has lower rotatory inertia and concomitant faster response.
Accurate measurement of fluid velocity depends upon the hydrodynamic properties of the fluid as it interacts with the rotating impeller. Thus, an accurate measurement of the fluid flow velocity is not obtained when the impeller and its supporting system are not designed to minimize the hydrodynamic disturbances it generates in the flowing fluid.